NANOINDENTATION OF BONE - Nanoindentation with Biological Applications - page 219

of nanoindentation or microindentation to quantify intrinsic, as opposed
to whole bone, mechanical properties enables the examination of the
bone tissue. 40,62,156-163 The evaluation of the mechanical properties of
both the whole bone and the intrinsic microscale properties are vital
to the understanding of the many bone disorders and their potential
treatments.
For example one study using mice that were genetically manipulated
to exhibit senile osteoporosis showed decreased whole bone bending
modulus and an increased indentation modulus. 62 While mechanical
properties of bone cannot be directly compared across the macroscopic
and microscopic scales, a similar trend might be expected as the whole
bone test inherently depends on the intrinsic microscale modulus of
bone as well as the bone hierarchical architecture. Such comparisons
warrant further investigation and demonstrate the need to probe bone's
mechanical properties at multiple hierarchical levels.
The use of labeling associated with nanoindentation provides a
practically interesting model that allows for site-specific tests located
on extant or newly-formed bone. Fluorescent makers, such as calcein
green or oxytetracycline, can be administered at specific time-
points during an experiment to label bone apposition at the time
of administration. 40,155,162,164,165 Microhardness testing on tail-suspended
mice, a common model for osteopenia and space-flight, showed
significantly reduced microhardness values in comparison to control
values. 166 Within the same bone samples, florescent labels enable
identification of extant versus newly formed bone to quantify the relative
bone changes that occur with a treatment. 158 Such techniques have been
minimally applied with nanoindentation, 158,167 yet have great potential.
4.2. Microstructural mechanical variation in human bone
A large number of nanoindentation studies have investigated the wide
range of mechanical properties found in various types and regions of
human bone. Nanoindentation's high load and displacement resolution is
ideally suited to investigate various microstructures in precise anatomical
locations. For identical testing methods and anatomical sites, the